Separate epitaxy layers for nanowire stack GAA device

ABSTRACT

The current disclosure describes techniques for forming gate-all-around (“GAA”) devices from stacks of separately formed nanowire semiconductor strips. The separately formed nanowire semiconductor strips are tailored for the respective GAA devices. A trench is formed in a first stack of epitaxy layers to define a space for forming a second stack of epitaxy layers. The trench bottom is modified to have determined or known parameters in the shapes or crystalline facet orientations. The known parameters of the trench bottom are used to select suitable processes to fill the trench bottom with a relatively flat base surface.

BACKGROUND

Complementary metal oxide semiconductor (CMOS) transistors are buildingblocks for integrated circuits. Faster CMOS switching speed requireshigher drive current, which drives the gate lengths of CMOS transistorsbeing continuously scaled down. Shorter gate length leads to undesirable“short-channel effects,” in which the current control function of thegates are compromised. FinFET transistors have been developed to, amongothers, overcome the short-channel effects. As a further step towardimproving the electrostatic control of the channels, transistors havingwrapped-around gates have been developed, in which a gate portion maysurround a semiconductor channel or a channel strip from the uppersurface, lower surface and/or sidewalls thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. In thedrawings, identical reference numbers identify similar elements or actsunless the context indicates otherwise. The sizes and relative positionsof elements in the drawings are not necessarily drawn to scale. In fact,the dimensions of the various features may be arbitrarily increased orreduced for clarity of discussion.

FIG. 1 is a cross-sectional view of an example integrated circuit (IC);

FIG. 2 is a flow diagram of an example process; and

FIGS. 3A-3J are cross-sectional views of a wafer in various stages of anexample process of making an IC having GAA devices.

DETAILED DESCRIPTION

The current disclosure describes techniques for forming gate-all-around(“GAA”) devices from stacks of separately formed nanowire semiconductorstrips. In the description herein, a “nanowire strip” or “nanowire”refers to a strip-shaped structure that has an edge surface dimensionranging from about 2 nm to about 50 nm. A “nanowire strip” may refer toa strip-shaped structure that has an edge surface diameter ranging fromabout 2 nm to about 15 nm. A “nanosheeo strip” may refer to astrip-shaped structure that have an edge surface having a height rangingfrom about 2 nm to about 10 nm and a width ranging from about 6 nm toabout 50 nm. Other dimensional parameters of the nanowire semiconductorstrips are also possible. In the description herein, the terms“nanowire” or “nanowire strip” or “strip” are used to refer to any typeof strip-shaped structures including, but not limited to, nanowire,nanosheet or nanoslab.

The separately formed nanowire semiconductor strips are tailored for therespective GAA devices. For example, the size, the material composition,and the positioning of strips are separately designed for nFET and pFETdevices. Between the nFET and pFET devices, at least one of the twotypes of devices are made from locally formed nanowire strips within adefine space. The inventors have observed that in the growth of epitaxylayers within a defined space, like in a trench, the profiles of thedeposited epitaxy layers are affected by the shaped of the bottomsurface of the defined space and the sidewall portions adjacent to thebottom surface. Specifically, the crystalline facets of the sidewallportions adjacent to the bottom surface affect the growth of the epitaxylayers by the sidewalls. In actual production, the shapes of trenchbottoms may be affected by many factors intentionally orunintentionally.

The current techniques include processes to intentionally modify orcreate a trench bottom portion to have determined or known parameters inthe shapes or crystalline facet orientations. With such parametersdetermined, the subsequent fabrication processes are conductedaccordingly.

For example, a crystallographic anisotropic wet etching is conducted toform a recess portion in a bottom of a trench. The depth of the recessportion and the slope angle of the sidewall of the recess portion aredetermined according to the anisotropic etching. With such parametersgiven, a silicon germanium epitaxy layer is formed in the recess portionto fill the recess portion. The silicon germanium epitaxy growth iscontrolled such that the silicon germanium material grows much fasteralong the facet orientation of the bottom plane of the recess portion,e.g., of {100} facet silicon, than along a facet orientation of thesidewall of the recess portion, e.g., of {111} facet of silicon. Forexample, the silicon germanium epitaxy process may be designed tofacilitate silicon germanium grown in the {100} facet instead of the{111} facet. Due to the lattice mismatch, the {100} facet silicongermanium does not stay on the sidewall of the recess portion of {111}facet silicon. With the silicon germanium epitaxy layer growsubstantially along the {100} facet in stead of the {111} face thereof,the silicon germanium epitaxy layer is substantially flat with respectto the bottom of the recess portion, e.g., of {100} facet silicon. Witha base silicon germanium epitaxy layer being substantially flat, theepitaxy layers of silicon and silicon germanium deposited over the basesilicon germanium epitaxy layer also tend to be flat.

In a method embodiment, a first stack of epitaxy silicon layers andepitaxy silicon germanium layers are formed over a substrate, e.g., asilicon substrate. The epitaxy silicon germanium layers and the epitaxysilicon layers are stacked vertically in an alternating manner. Asilicon germanium layer in the first stack has a first thickness and afirst germanium concentration, e.g., an atomic ratio of germanium amongtotal silicon and germanium. A silicon layer in the first stack has asecond thickness that may be different than the first thickness of thesilicon germanium in the first stack. In an embodiment, the siliconlayers and the silicon germanium layers in the first stack are formedusing epitaxy processes and are referred to as epitaxy layers.

A trench is formed at least partially in the first stack of epitaxylayers, which exposes the silicon substrate. Optionally, a device spacerof dielectric material is formed by the sidewall of the trench. Afterthe formation of the trench or after the formation of the device spacer,a bottom surface of the trench may include a recess in the siliconsubstrate. Normally, the recess includes a first portion which is arelatively flat bottom of the recess. A second portion of the recess isa sloped sidewall portion adjacent to the flat bottom. The actual shapesor parameters of the recess may vary among wafers.

A crystallographic anisotropic etching is conducted to modify the shapeof the recess portion. Specifically, the modification is achievedthrough the crystallographic anisotropic etching process has differentetching rates among different crystalline facets/planes of thesubstrate. For example, the crystallographic anisotropic etch process iscontrolled to have higher etch rates on the {110} {100} facets than onthe {111} facets. Resultantly, the sidewall of the recess portion ismodified to be substantially at the {111} facet of the siliconsubstrate. For the silicon, the {111} facet has an angle of about 54.7degree with respect to the {100} plane. After the modification, thesidewall of the recess portion has an angle of about 54.7 degree withrespect to the bottom portion. Other angles of the sidewall are alsopossible as long as the angles are known and controlled. For example,the angle of the sidewall may be formed within a range of about 45degree to about 65 degree to be generally in line with the angle betweenthe {111} facet plane and the {100} plane of silicon.

After the recess portion is modified, a base layer of silicon germaniumis formed in the recess portion through an epitaxy process. Silicongermanium has a good gap filling property such that the minor unevennesson the bottom of the recess may be filled by the silicon germanium baselayer. Further, the epitaxy conditions may be selected and controlledsuch that the silicon germanium material does not form on the {111}facet, e.g., due to lattice mismatch. As a result, the silicon germaniumbase layer is substantially facet-free at the edge portions and isrelatively flat. Depending on the stack of epitaxy layers to be formedwithin the trench, an epitaxy base layer of silicon may be formed overthe silicon germanium base layer. Given that the surface of the silicongermanium base layer is relatively flat, the surface of the silicon baselayer is also relatively flat. The top surface of the silicon base layeris substantially at a same level as the silicon substrate. The silicongermanium and silicon base layers are used to fill the recess and togenerate a flat surface for subsequent deposition of epitaxy layerswithin the trench. They are not used to form a semiconductor body, e.g.,a channel of a device.

A second stack of silicon epitaxy layers and silicon germanium epitaxylayers are formed in the trench over the base layers. In an embodiment,a silicon germanium epitaxy layer in the second stack includes a thirdthickness and a second germanium concentration. A silicon epitaxy layerin the second stack includes a fourth thickness. In an embodiment, thethird thickness of the silicon germanium layer in the second stack isdifferent from the first thickness of the silicon germanium layer in thefirst stack. The fourth thickness of the silicon layer in the secondstack is different from the second thickness of the silicon layer in thefirst stack. Further, the second germanium concentration of the silicongermanium layer in the second stack is different from the firstgermanium concentration of the silicon germanium layer in the firststack.

The first stack of epitaxy layers and the second stack of epitaxy layersare patterned to define a first fin region and a second fin region for afirst device and a second device. The first device is formed over thefirst fin region. The second device is formed over the second finregion. The silicon germanium base layer and the silicon base layer mayalso be patterned below the second fin region. The silicon germaniumbase layer has an inverted tapered sidewall that has an angle of about54.7 degree with a bottom surface of the silicon germanium layer.

The following disclosure provides many different embodiments, orexamples, for implementing different features of the described subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present description. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

In the following description, certain specific details are set forth inorder to provide a thorough understanding of various embodiments of thedisclosure. However, one skilled in the art will understand that thedisclosure may be practiced without these specific details. In otherinstances, well-known structures associated with electronic componentsand fabrication techniques have not been described in detail to avoidunnecessarily obscuring the descriptions of the embodiments of thepresent disclosure.

Unless the context requires otherwise, throughout the specification andclaims that follow, the word “comprise” and variations thereof, such as“comprises” and “comprising,” are to be construed in an open, inclusivesense, that is, as “including, but not limited to.”

The use of ordinals such as first, second and third does not necessarilyimply a ranked sense of order, but rather may only distinguish betweenmultiple instances of an act or structure.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment. Thus, the appearances of the phrases “in one embodiment” or“in an embodiment” in various places throughout this specification arenot necessarily all referring to the same embodiment. Furthermore, theparticular features, structures, or characteristics may be combined inany suitable manner in one or more embodiments.

As used in this specification and the appended claims, the singularforms “a,” “an,” and “the” include plural referents unless the contentclearly dictates otherwise. It should also be noted that the term “or”is generally employed in its sense including “and/or” unless the contentclearly dictates otherwise.

The gate all around (GAA) transistor structures may be patterned by anysuitable method. For example, the structures may be patterned using oneor more photolithography processes, including double-patterning ormulti-patterning processes. Generally, double-patterning ormulti-patterning processes combine photolithography and self-alignedprocesses, allowing patterns to be created that have, for example,pitches smaller than what is otherwise obtainable using a single, directphotolithography process. For example, in one embodiment, a sacrificiallayer is formed over a substrate and patterned using a photolithographyprocess. Spacers are formed alongside the patterned sacrificial layerusing a self-aligned process. The sacrificial layer is then removed, andthe remaining spacers may then be used to pattern the GAA structure.

FIG. 1 is a cross-sectional view of an example integrated circuitstructure “IC” 100 including a CMOS component. Referring to FIG. 1, theIC 100 includes a substrate 102. Optionally, the substrate 102 includesan n-doped region “n-well” 104 and a p-doped region “p-well” 106. Afirst device 110, e.g., a pFET, is formed over and/or within the n-well104. A second device 150, e.g., an nFET, is formed over and/or withinthe p-well 106. One or more dielectric body 108 (also referred to asdevice spacer 108) is positioned to separate the first device 110 andthe second device 150 from one another. It should be appreciated thatdepending on device designs or configurations, the n-well 104 or thep-well 106 may not be needed for the substrate 102. In the descriptionherein, the n-well 104 and the p-well 106 are used for illustration andare referred to as substrate region 104 and substrate region 106 forpurposes of generalization.

The first device 110 and the second device 150 each includes a verticalstack of a plurality of (four shown for each for illustration purposesonly) discrete semiconductor nanowire strips 120, 160, respectively. Inan embodiment, the nanowire strips 120 are silicon germanium and thenanowire strips 160 are silicon. The first device 110 and the seconddevice 150 each includes a gate structure 130, 170 that surrounds, e.g.,wraps around, at least some of the respective discrete nanowire strips120, 160. The discrete nanowire strips 120, 160 are configured aschannel regions of the devices 110, 150 and form junctions withrespective source/drain regions 140, 180. The source/drain regions 140,180 are separated from the respective gates 130, 170 by inner spacers134, 174 and/or outer spacers 132, 172.

In an embodiment, the device 110 is configured as a pFET and the device150 is configured as an nFET. The nanowire strips 120 are silicongermanium or other suitable semiconductor materials. The source/drainregion 140 is silicon germanium (“SiGe”) or silicon-germanium-boron(“SiGeB”), or other suitable semiconductor materials for P-type devices.The source/drain region 180 is silicon-carbon-phosphide (“SiCP”),silicon carbide (“SiC”), or silicon phosphide (“SiP”), or other suitablesemiconductor materials for N-type devices. The source/drain regions140, 180 may each be doped in various ways. For example the source/drainregion 140 are doped with boron, gallium, indium and other suitabledopants in group III. The source/drain regions 180 are doped witharsenic, phosphorus and other suitable dopants in group V.

FIG. 1 shows example embodiments of the source/drain structures 140,180. In the embodiment shown, the source/drain structure 140, 180 eachcontacts the respective nanowire strips 120, 160 by the edge surfaces ofthe nanowire strips 120, 160. Further, the source/drain structure 140,180 are each adjacent to a respective dielectric layer 142, 182. Thedielectric layers 142, 182 may be a same layer or may be two differentlayers. The dielectric layers 142, 182 may include the same dielectricmaterial as the device spacer 108 or may include different dielectricmaterials from that of the device spacer 108. The dielectric layers 142,182 may be silicon oxide, silicon nitride, a low-k dielectric materialor other suitable dielectric materials.

FIG. 1 shows, as an illustrative example, an example structuralconfiguration between the source/drain regions 140, 180 and the channelregions 120, 160. Specifically, the source/drain region 140, 180 eachcontacts the edge surfaces of the respective channels 120, 160. Thisexample embodiment does not limit the scope of the disclosure and otherembodiments. Other structural configurations of the source/drainstructure 140/180 and the semiconductor nanowire strips 120/160 are alsopossible and included in the disclosure. For example, the source/drainstructures 140/180 may wrap around at least some of the respectivesemiconductor nanowire strips 120/160. The nanowire strips 120, 160 maybe receded (as shown in FIG. 1) or may extend all the way between therespective dielectric layers 142, 182.

The substrate 102 may include a silicon substrate in crystallinestructure and/or other elementary semiconductors like germanium.Alternatively or additionally, the substrate 102 may include a compoundsemiconductor such as silicon carbide, gallium arsenide, indiumarsenide, and/or indium phosphide. Further, the substrate 102 may alsoinclude a silicon-on-insulator (SOI) structure. The substrate 102 mayinclude an epitaxial layer and/or may be strained for performanceenhancement.

In an embodiment, the substrate 102 include a facet region of the {100}facet of silicon.

In an embodiment, the gate structures 130, 170 are each formed as areplacement metal gate. The following description lists examples ofmaterials for the gate structure 130, 170. The gate electrodes of thegates 130, 170 (not separately shown for simplicity) each include aconductive material, e.g., a metal or a metal compound. Suitable metalmaterials for the gate electrode of the gate structures 130, 170 includeruthenium, palladium, platinum, tungsten, cobalt, nickel, and/orconductive metal oxides and other suitable P-type metal materials andinclude hafnium (Hf), zirconium (Zr), titanium (Ti), tantalum (Ta),aluminum (Al), aluminides and/or conductive metal carbides (e.g.,hafnium carbide, zirconium carbide, titanium carbide, and aluminumcarbide), and other suitable materials for n-type metal materials. Insome examples, the gate electrodes of the gate structures 130, 170 eachinclude a work function layer tuned to have a proper work function forenhanced performance of the field effect transistor devices. Forexample, suitable n-type work function metals include Ta, TiAl, TiAlN,TaCN, other n-type work function metal, or a combination thereof, andsuitable p-type work function metal materials include TiN, TaN, otherp-type work function metal, or combination thereof. In some examples, aconductive layer, such as an aluminum layer, a copper layer, a cobaltlayer or a tungsten layer is formed over the work function layer suchthat the gate electrodes of gate structures 130, 170 each includes awork function layer disposed over the gate dielectric and a conductivelayer disposed over the work function layer and below a gate cap (notshown for simplicity). In an example, the gate electrodes of the gatestructures 130, 170 each have a thickness ranging from about 5 nm toabout 40 nm depending on design requirements.

In example embodiments, the gate dielectric layer (not separately shownfor simplicity) of the gate structures 130, 170 includes an interfacialsilicon oxide layer (not separately shown for simplicity), e.g., thermalor chemical oxide having a thickness ranging from about 5 to about 10angstrom (Å). In example embodiments, the gate dielectric layer furtherincludes a high dielectric constant (high-k) dielectric materialselected from one or more of hafnium oxide (HfO₂), hafnium silicon oxide(HfSiO), hafnium silicon oxynitride (HfSiON), hafnium tantalum oxide(HMO), hafnium titanium oxide (HMO), hafnium zirconium oxide (HfZrO),combinations thereof, and/or other suitable materials. A high-kdielectric material, in some applications, includes a dielectricconstant (k) value larger than 6. Depending on design requirements, adielectric material of a dielectric contact (k) value of 7 or higher isused. The high-K dielectric layer may be formed by atomic layerdeposition (ALD) or other suitable technique. In accordance withembodiments described herein, the high-k dielectric layer of the gatedielectric layer includes a thickness ranging from about 10 to about 30angstrom (Å) or other suitable thickness.

In an embodiment, the outer spacers 132, 172 is formed of a low-kdielectric material, e.g., k value lower than 3.9, such as siliconoxynitride (SiON), silicon monoxide (SiO), silicon oxynitrocarbide(SiONC), silicon oxycarbide (SiOC), vacuum, and other dielectrics orother suitable materials. The outer spacer 132, 172 may be formedthrough chemical vapor deposition (CVD), high density plasma CVD,spin-on, sputtering, or other suitable approaches.

In an embodiment, the inner spacers 134, 174 are formed of a low-Kdielectric material. In some embodiments, the low-k dielectric materialof the inner spacers 134, 174 may have a different dielectric constantthan that of the respective outer spacer 132, 172. The low-k materialfor the inner spacer 134, 174 includes one or more of silicon oxynitride(SiON), silicon monoxide (SiO), silicon oxynitrocarbide (SiONC), siliconoxycarbide (SiOC), or other suitable low-k dielectric material. In someembodiments, the k value of the inner spacer 134, 174 materials is thesame as the k value of the outer spacer 132, 172 material. In anexample, the inner spacer 134, 174 also includes one or more air gapadjacent to one or more of the respective gate structure 130, 170 or therespective source/drain structure 140, 180.

In an embodiment, a base layer 190 is positioned within the substrate102 and below one or more of the device 110 or the device 150. FIG. 1shows, as an illustrative example, that the base layer 190 is positionedwithin the substrate region 104 and below the device 110, which does notlimit the scope of the disclosure. In an embodiment, the base layer 190includes a silicon germanium base layer 192 that is formed directly overthe underlying substrate 102, e.g., the N-well 104. In some scenarios, asilicon base layer 194 is formed over the silicon germanium base layer192, which are both parts of the base layer 190. In some embodiments,the base layer 190 may include multiple silicon germanium base layersand multiple silicon base layers vertically stacked in an alternatingmanner. In an embodiment, a surface 190S of the base layer 190 issubstantially at a same level as a surface 102S of the substrate 102.

In an embodiment, the base layer 190 includes a bottom portion 190B anda sloped sidewall portion 190SW. An angle θ1 between the bottom portion190B and the sloped sidewall portion 190SW is within a range betweenabout 5 degree to about 89 degree. In an embodiment, the angle θ1 iswithin a range between about 45 degree to about 65 degree. In a furtherembodiment, in a case that the substrate 102 is crystalline silicon, theangle θ1 is substantially about 54.7 degree.

A thickness T1 of the silicon germanium base layer 192 ranges betweenabout 2 nm to as large as a critical thickness of the silicon germaniummaterial of the silicon germanium base layer 192. The critical thicknessof a silicon germanium material depends on a germanium concentration ofthe silicon germanium material. Critical thickness of Si_(1-x)Ge_(x)refers to a maximum thickness of Si_(1-x)Ge_(x) that can be grown overSi before relaxation of the strain occurs through the formation ofmisfit dislocations. The critical thickness of Si_(1-x)Ge_(x) dependsstrongly on the germanium content, i.e., the “x”. Specifically, a highergermanium concentration leads to a lower critical thickness of thesilicon germanium material. For example, in a case that the germaniumconcentration of the germanium base layer 190 is about 20%, thethickness T1 is within a range between about 2 nm to about 60 nm.

The nanowire strips 120 and the nanowire strips 160 are formed fromstacks of epitaxy layers that are deposited separately in differentepitaxy growth procedures. In an embodiment, the epitaxy layers for thenanowire strips 160 are deposited in a first region while the epitaxylayers for the nanowire strips 120 are deposited in a second regioncontained within the first region. Specifically, the epitaxy layers forthe nanowire strips 120 is formed in a trench formed in the epitaxylayers for the nanowire strips 160. The deposition procedures aredescribed herein in details. The nanowire strips 120 and the nanowirestrips 160 may include different thickness. Because the sacrificialstrips used in making one of the devices 110, 150 and the nanowirestrips 120, 160 of the other one of the first device 110 or the seconddevice 150 are not made from the same epitaxy layers, they do notnecessarily align with one another. As such, an inner spacer 134, 174 ofone of the devices 110, 150 may not laterally align with a nanowirestrip 160, 120 of the other one of the devices 150, 110. Note again thatthe nanowire strips 120 and the nanowire strips 160 are formed fromstacks of epitaxy layers that are deposited separately in differentepitaxy procedures as described herein in more details.

FIG. 1 shows that the n-well 104 and the p-well 106 are formed adjacentto one another, which is not limiting. In other embodiments, the p-well106 and the n-well 104 may be separated by one or more insulationbodies, e.g., shallow trench insulation (“STI”). FIG. 1 shows that adual-tub process is used, i.e., both p-well 106 and n-well 104 areformed in the substrate 102. Other processes, like a p-well process inan n-type substrate or an n-well process in a p-type substrate are alsopossible and included in the disclosure. That is, it is possible thatone of the substrate regions 104, 106 is in a doped local region and theother is in the doped substrate. It is also possible that both thesubstrate region 104 and the substrate region 106 are intrinsic orintrinsically doped, e.g., unintentionally doped.

FIG. 2 shows an example process of making an IC device, e.g., theexample IC 100 of FIG. 1. FIGS. 3A-3J show in cross-sectional views,different stages of a wafer 300 in the process of making the example IC100 according the example process of FIG. 2.

In example operation 205, a wafer 300 is received. FIG. 3A shows thatwafer 300 includes a substrate 102 that includes substrate region 104,e.g., an n-well, and a substrate region 106, e.g., a p-well, which arecoplanar with one another at an upper surface 102S of the substrate 102.As an illustrative example, the substrate 102 is a silicon substrate inthe crystalline facet of {100}. FIG. 3A shows, as a non-limitingillustrative example, that the substrate region 104 and the substrateregion 106 interface with one another. The n-well 104 may be doped invarious approaches with dopants or impurities of group V elements, likearsenic or phosphorous, or various combinations thereof. The p-well 106may be doped in various approaches with dopants or impurities of groupIII elements, like boron or gallium, or various combinations thereof.

In example operation 210, with reference also to FIG. 3B, a first stack310 of semiconductor epitaxy layers are formed globally on top of boththe substrate region 104 and the substrate region 106. The term“globally” refers to that the first stack 310 are formed on both thesubstrate regions 104 and 106 as compared to “locally” to one of thesubstrate region 104 or the substrate region 106. The term “globally” asused herein does not necessarily mean that the first stack 310 is formedover the whole surface of the wafer. In an embodiment, the first stack310 includes multiple epitaxy layers 312 (four layers shown as anillustrative example) of a first semiconductor material and multipleepitaxy layers layer 314 (four layers shown as an illustrative example)of a second semiconductor material. In an embodiment, layers 312 aresilicon germanium and layers 314 are silicon.

In an embodiment, the silicon germanium epitaxy layers 312 each have athickness T2 that is in a range about 2 nm to about 6 nm. The siliconepitaxy layers 314 each have a thickness T3 that is in a range about 4nm to about 10 nm. In an embodiment, the thickness T3 of the siliconepitaxy layers 314 is larger than the thickness T2 of the silicongermanium epitaxy layers 312. In an embodiment, the thickness T3 of thesilicon epitaxy layers 314 is about 8 nm and the thickness T2 of thesilicon germanium epitaxy layers 312 is about 5 nm.

The epitaxy layers 312, 314 of the first stack 310 are formed usingreduced pressure chemical vapor deposition (“RP-CVD”), plasma enhancedchemical vapor deposition (“PECVD”), atmospheric pressure CVD,inductively coupled PECVD, hot wire CVD, atomic layer deposition,molecular layer deposition or other suitable epitaxy approaches.

In example operation 215, with reference also to FIG. 3C, a trench 320is formed through the first stack 310 over the first substrate region104. For example, an oxide mask layer or a photoresist layer 321 isformed and patterned to expose the surface for forming the trench 320.The trench 320 exposes at least a portion of the first substrate portion104. In an embodiment, the trench 320 is formed through a dry etchingprocess, such as reactive-ion etching (“RIE”) or other suitable dryetching processes. As the RIE is directional, the sidewalls 320SW of thetrench 320 are relatively plumb. It is desired that the bottom 320B ofthe trench 320 is relatively flat. However, in actual production, theRIE may produce a bottom 320B that includes a relatively flat centralbottom portion 320BB and sloped bottom sidewall portion 320BS. In FIG.3C, the central bottom portion 320BB and the sloped bottom sidewallportion 320BS are illustrated using dotted lines.

In example operation 220, with reference also to FIG. 3D, a conformaldielectric layer 322 is deposited over the wafer 300 including the firststack 310 and the trench 320. The dielectric layer 322 is oxide, nitrideor other suitable dielectric material and is formed through CVD, ALD orother suitable deposition techniques. The deposition thickness of thedielectric layer 322 is about 5 nm to about 60 nm. It is also possiblethat the dielectric layer 322 is deposited within the trench 320 onlyand is not deposited over the first stack 310. In an embodiment, themask layer 321 remains covering the first stack 310.

In example operation 225, with reference also to FIG. 3E, device spacer108 is formed between the first stack 310 and the trench 320.Specifically, the device spacer 108 laterally separates the trench 320from the first stack 310. The device spacer 108 is formed by patterningthe dielectric layer 322. The dimension(s) of the device spacer 108 maychange in the subsequent processes, e.g., after the mask layer 321 isremoved later, which is appreciated in the field of semiconductor waferfabrications.

With the device spacer 108 formed, the bottom 320B of the trench 320 maybe further impacted by the etching and the bottom 320B may actuallyinclude a relatively flat bottom portion 320BB and a sloped bottomsidewall portion 320BS, shown in dotted line.

In example operation 230, with reference also to FIG. 3F, a recessportion 330 is formed by modifying the bottom 320B of the trench 320. Inan embodiment, the modification is achieved through an anisotropicetching that has different etch rates for different crystalline facetorientations of the substrate 102 material. For example, in the casethat the substrate 102 is silicon, one or more etchants of ammonia, HCl,KOH, TMAH or EDP may be used to implement the crystallographicanisotropic etching. For example, for ammonia, the relative etchingrates among various silicon facets are {110}>{100}>>{110}. For HCl, theetching rate differences among {100}, {110}, {111} facets are 10:78:9.For KOH, the etching rate differences among {100}, {110}, {111} facetsare 300:600:1. For TMAH, the etching rate differences among {100},{110}, {111} facets are 37:68:1. For EDP, the etching rate differencesamong {100}, {110}, {111} facets are 20:10:1. With the suitable etchantsapplied in the etching process, a corresponding profile of the recessportion 330 is formed. For example, in an example that the substrate 102is {100} facet of silicon, the KOH or TMAH etchants will form a recessportion 330 that has a relatively flat bottom portion 330B, and a slopedsidewall portion 330BS that is substantially in the {111} facet ofsilicon. This shape is achieved through the vast differences in theetching rate among {100}, {110} and {111} facets of silicon. Forexample, in an embodiment, an angle θ2 between the sidewall 330BS andthe bottom 330B of the recess portion 330 is substantially about 54.7degree, which is basically the angle between the {111} facet plane tothe {100} facet plane of silicon.

With different etchants chosen and different etching conditions, e.g.,etchant solution concentration, temperature, pressure, etc., theprofiles or shapes of the recess portion 330 vary. For example, theangle θ2 may vary between about 5 degree to about 89 degree. This anglerange is important to maintain that the sidewall 330BS is sloped and isnot plumb/vertical, e.g., 90 degree. A depth D1 of the recess portion330, e.g., from the bottom portion 330B to the surface of the substrate102, may also vary from about 2 nm to about 80 nm. However, all thosevariations are controlled or controllable variation. As such, theresultant profile or shape of the recess portion 330, e.g., parametersof the angle θ2 and/or the depth D1, is relatively determined or knownthrough simulation or experimental data. These known parameters of theresultant recess portion 330 enables techniques to fill the recessportion 330 in a proper way such that a surface of the filler layer orlayers is relatively flat.

In an embodiment, because the anisotropic etching process is conductedafter the device spacer 108 is formed, a portion of the substrate 102below the device spacer 108 is removed by the etching process.Therefore, an undercut is formed and an edge 330E of the recess portion330 extends below the device spacer 108.

In example operation 235, with reference also to FIG. 3G, asemiconductor base layer 340 is formed within and filling the recessportion 330. In an embodiment, the semiconductor base layer includes asemiconductor base layer of a material that has gap filling properties.For example, silicon germanium is a suitable semiconductor material forthe gap filling properties. In an embodiment, the semiconductor baselayer 340 includes one or more silicon germanium layers 342, with one ofthe silicon germanium layer 342 directly on the underlying substrate102. That is, minor gaps on the bottom 330B of the recess portion 330,if any, are filled by the silicon germanium layer 342, which are nottranslated to the upper surface 342U of the silicon germanium base layer342.

In an embodiment, an epitaxy process is used in depositing the silicongermanium base layer 342. The epitaxy process is tuned in a way that thesilicon germanium material grows faster along the facet orientation 346of the bottom 330B than along the facet orientation 348 of the sidewall330BS of the recess portion 330. As a result, the epitaxy growth alongthe facet orientation 348 is refrained and the silicon germanium baselayer 342 is deposited substantially along the orientation 346 and has arelatively flat upper surface 342U. For example, in a case that thesloped sidewall 330B S is at the {111} facet plane of silicon and thebottom 330B is at the {100} facet plane of silicon, the silicongermanium epitaxy process is tuned to deposit {100} facet silicongermanium. Due to lattice mismatch, the grown silicon germanium of {100}facet does not stay on the sloped sidewall 330BS of {111} facet silicon.For example, the epitaxy process may include temperatures in a rangebetween about 500° C. to about 650° C., a pressure range between about10 Torr to 300 Torr. The precursors may be one or more of SiH₂Cl₂(“dichlorosilane”) and GeH₄ (“germane”), SiH₄, Si₂H₆ and Ge₂H₆, or othersuitable precursors for silicon and germanium.

It should be appreciated that the silicon germanium base layer epitaxyprocess is tuned based on the known or determined parameters of therecess 330, e.g., the angle θ2 of the sidewall 330BS. When the angle θ2is not exactly 54.7 degree, e.g., the {111} facet, the facet orientation348 of the sidewall 330BS may include components of the {100} facet, the{110} facet and the {111} facet. The fine tuning of the epitaxy processdetermines a suitable set of epitaxy conditions to achieve the selectivegrowth of the silicon germanium layer 342 along the facet orientation346 instead of the facet orientation 348. Further, the example operation235 and the example operation 230 may also be coordinated in a reversedway in that the angle θ2 of the sidewall 330BS is determined based on afixed epitaxy process, e.g., with known silicon germanium depositionrates among different facet orientations. With that, a suitablecrystallographic anisotropic etching process is selected to form therecess portion 330 with the determined angle θ2 suitable for the epitaxyprocess.

With this technique being applied, the bottom profile of the trench 320is modified or determined. The parameters like the angle θ2 and thedepth D1 are obtained through simulation or experiments. With suchparameters provided, the epitaxy process of forming the silicongermanium base layer 342 is controllable to achieve a relatively flatupper surface 342U.

Further, with the depth D1 of the recess portion 330 provided, thedeposition thickness of the silicon germanium epitaxy layer 342 isaccurately designed and may be monitored in-situ during the epitaxyprocess through, e.g., oscillating quartz resonators. Such in-situmonitoring provides substantially real-time feedback to the thicknesscontrol in the thin film deposition component of the epitaxy process.Therefore, precise control of the thickness of the epitaxy layer 342 isenhanced by optimizing the epitaxy parameters dynamically during thedeposition process through the real time monitoring and feedbackapproaches.

Additionally, the structural feature of the edge 330E extending belowthe device spacer 108 provides an aspect ratio change benchmark for thesilicon germanium epitaxy layer 342. With the aspect ratio changebenchmark, the epitaxy process is controlled such that the silicongermanium epitaxy layer 342 or any additional epitaxy layer of the baselayer 340 stop at the edge 330E or at the lower surface 108L of thedevice spacer 108. As a result, an upper surface 340U is substantiallyat a same level as the upper surface 102U of the substrate 102 where thedevice spacer 108 seats on. Further, an edge portion 340E of the baselayer 340 extends below the lower surface 108L of the device spacer 108.

In an embodiment, depending on the material of the epitaxy layers thatis formed over the semiconductor base layer 340, another base layer 344may be formed over the silicon germanium base layer 342. For example, asilicon base layer 344 may be formed over the silicon germanium baselayer 342. Similar controls of epitaxy growth on the facet direction 348of the sidewall 330BS and on the facet direction 346 of the bottom 330Bare applied to the deposition of the silicon base layer 344. As aresult, the upper surface of the silicon base layer 344 is relativelyflat.

FIG. 3G shows, as an illustrative example, that the semiconductor baselayer 340 includes two layers 342 and 344 of silicon germanium andsilicon, respectively. This specific example does not limit the scope ofthe disclosure. The base layer 340 may include other semiconductormaterials suitable for selective epitaxy growth along different facetdirections. Further, the semiconductor base layer 340 may include morethan two layers of different base layers depending on the depth D1 ofthe recess portion 330, the critical thickness of a base layer, and thematerials of the epitaxy layer directly grown on the upper surface 340Uof the base layer 340. For example, a silicon germanium base layer 342should be thinner than the critical thickness thereof to maintainstructural integrity. The critical thickness of a silicon germaniumlayer depends on a germanium concentration thereof. For example, in acase that the germanium concentration is about 20%, the criticalthickness is about 60 nm.

In an embodiment, the forming the base layer 340 includes a pre-bakingprocess 238, which heats the recess portion 330 before the epitaxyprocess of the base layer 340. The pre-baking process removes the nativeoxide on the bottom of the recess portion 330. In an embodiment, thepre-baking process is conducted with a surface temperature of 800-950°C., a surface pressure of 100 Torr-300 Torr and with a flow of hydrogenfor about 10-100 seconds.

In example operation 240, with reference also to FIG. 3H, a second stack350 of epitaxy layers 352, 354 are formed over the base layer 340 andwithin the trench 320. The second stack 350 includes silicon germaniumepitaxy layers 352 and silicon epitaxy layers 354 stacked vertically inan alternating manner. In an embodiment, a thickness T4 of the silicongermanium layer 352 in the second stack 350 is different from thethickness T2 of the silicon germanium layer 312 in the first stack 310.A thickness T5 of the silicon layer 354 in the second stack 350 isdifferent from the thickness T3 of the silicon layer 314 in the firststack 310. Further, the thickness T4 of the silicon germanium layer 352is different from the thickness T5 of the silicon layer 354 in thesecond stack 350.

Further, as the silicon germanium layers 352 in the second stack 350 areformed separately from the silicon germanium layer 312 of the firststack 310, the silicon germanium layers 352 may include a differentgermanium concentration than the silicon germanium layer 312.

As shown in FIG. 3H, after the second stack 350 is formed. The masklayer 321 (FIG. 3G) may be removed through, e.g., etching orplanarization, and the device spacer 108 may be truncated to reduce thevertical dimension through the planarization process.

In an embodiment, the thickness T2, T3, T4, T5 are individually designedfor the first stack 310 and the second stack 350. Factors may includewhether the respective epitaxy layers 312, 314, 352, 354 are used toform channel nanowire strips or as sacrificial nanowire strips. Factorsmay also include the respective types of devices to be formed therefrom.The first stack 310 and the second stack 350 are separately formed tomake different types of devices, e.g., nFET and pFET devices. Using thedisclosed techniques, the first stack 310 and the second stack 350 areseparately formed and the thickness T2, T3, T4, T5 of the epitaxy layers312, 314, 352, 354 are separately controlled to meet the individualdevice designs or operation designs for the respective devices, e.g.,nFET and pFET.

In an embodiment, to facilitate the subsequent fabrication processes,the upper surface 310U and 350U of the first stack 310, the second stack350, respectively, are substantially at a same level. The thickness T4of the silicon germanium layer 352 of the second stack 350 issubstantially the same as the thickness T3 of the silicon layer 314 ofthe first stack 310, here, e.g., 8 nm. The thickness T5 of the siliconlayer 354 of the second stack 350 is substantially the same as thethickness T2 of the silicon germanium layer 312 of the first stack 310,here, e.g., 5 nm.

FIG. 3H shows that the first stack 310 and the second stack 350 includea same sequential order among the respective silicon germanium epitaxylayers 312, 352 and the silicon epitaxy layers 314, 354. That is, thestacks 310, 350 both start with a silicon germanium epitaxy layers 312,352 at the bottom and end with a silicon epitaxy layers 314, 354 at thetop. This illustrative example does not limit the scope of thedisclosure. One or more of the first stack 310 or the second stack 350may start with a silicon epitaxy layer at the bottom, which is alsopossible and included in the disclosure.

FIG. 3H shows, as an illustrative example, that the second stack 350 andthe first stack 310 both include silicon and silicon germanium epitaxylayers. This example does not limit the scope of the disclosure. Inother examples, the second stack 350 may include other materialcombinations for the epitaxy layers from that of the first stack 310.

In a GAA process, the silicon epitaxy layers 314 of the first stack 310are used to make nanowire channel regions of nFET devices and thesilicon germanium epitaxy layers 312 of the first stack 310 are used tomake sacrificial nanowire strips, e.g., to be removed later. The silicongermanium epitaxy layers 352 of the second stack 350 are used to makenanowire channel regions of pFET devices and the silicon epitaxy layers354 of the second stack 350 are used to make sacrificial nanowirestrips.

In example operation 245, with reference also to FIG. 3I, the firststack 310 and the second stack 350 are patterned to form a firstpatterned stack 310P of silicon germanium nanowire strips 312 andsilicon nanowire strips 314 and a second patterned stack 350P of silicongermanium nanowire strips 352 and silicon nanowire strips 354. In someembodiment, the first patterned stack 310P and the second patternedstack 350P are fin-shaped.

In some embodiment, the patterning may also form fin-shaped substrateportions 106P, 104P (both shown in dotted lines) below the fin-shapedfirst patterned stack 310P and the second patterned stack 350P,respectively. Dielectric layers may be formed surrounding the fin-shapedsubstrate portion 106P, 104P up to the level of the upper surface 106U,340U of the second substrate region 106, the base layer 340,respectively. The fin-shaped substrate portion 104P includes a patternedportion 196 (FIG. 1) of the base layer 340.

In example operation 250, with reference also to FIG. 3J, an nFET device150 and a pFET device 110 are formed over the first patterned stack 310Pand the second patterned stack 350P, respectively, using agate-all-around (“GAA”) process. For example, the channels 160 of thenFET device 150 are made from the silicon nanowire strips 314, and thechannels 120 of the pFET device 110 are made from the silicon germaniumnanowire strips 352. The sacrificial nanowire strips 312 of silicongermanium is removed and replaced with the gate structure 170 of thenFET device 150. The sacrificial nanowire strips 354 of silicon isremoved and replaced with the gate structure 130 of the pFET device 110.

The base silicon germanium layer 192 (patterned from base layer 342) andthe base silicon layer 194 (patterned from base layer 344) are part ofthe patterned substrate 104P below the pFET device 110. The base silicongermanium layer 192 and the base silicon layer 194 are not used to makethe channel region 120 because their deposition are tailored for fillingthe recess portion 330 and are not tailored for the channels region(s).

Because the edge portion 340E of the base layer 340 extend below thedevice spacer 108, at least part of the edge portion 340E will remainafter patterning the substrate 104 and forming the fin-shaped substratepattern 104P. In an embodiment, the remaining edge portions 340E areadjacent to the device spacer 108 on the top and the dielectric layer360 by the side. The remaining edge portions 340E are part of the baselayer 190, which are both patterned from the same base layer 340.

As appreciated, FIG. 3J shows the devices 110, 150 from a different viewfrom that of FIG. 1. Either one or both of FIGS. 1 and 3J are not meantto limit the scope of the disclosure with respect to the relativepositions and/or connection arrangements between the nFET device 150 andthe pFET device 110 in a complementary manner in an integrated circuit.

As described herein, the sequential orders, the materials, and thethickness or other parameters of the nanowire strips 312, 314, 352, 354in the separately formed nanowire stack 310, 350 could be customized andoptimized separately for each devices 150, 110. Such flexibility isadvantageous in improving the device performance of pFET and nFETdevices separately and individually. Further, the device spacer 108 isintegrated in the formation of the nanowire stacks 310, 350, whichsimplifies the integration of the CMOS process into a high voltageanalog process like a bipolar-CMOS-DMOS “BCD” process.

Further, the disclosed technique of forming the base layer 340 resolvesthe problems of epitaxy growth in a sloped bottom portion of a trench.The controlled facet angles of the recess portion 330 and the controlledepitaxy growth of the base layers in the base layer 340 achieve arelatively flat upper surface of the base layer 340. Such a base layer340 facilitates epitaxy growth of semiconductor layers thereover.

In an alternative or additional embodiment to FIG. 3C, a hard mask layerstays over the first stack 310 after the trench 320 is formed. Forexample, the hard mask layer may be is patterned as an etch stop layerin etching the trench 320. After the trench 320 is formed throughetching, the hard mask layer is not removed or is not lately removed.The remaining hard mask layer may function as an additional etch stoplayer or CMP stop layer in a subsequent process that includes an etchingcomponents or a subsequent CMP process. For example, a subsequentepitaxy process may include an etching component. The hard mask layer isone or more of silicon oxide, aluminum hafnium oxide, magnesium aluminumoxide, silicon nitride or other suitable etch stop materials.

The hard mask layer functions as an etch stop or CMP stop layer inremoving the excess dielectric layer 322 that is deposited over thefirst stack 310.

The hard mask layer may remain until the second stack 350 is formedwithin the trench 320, e.g., through processes similar to those shown inFIGS. 3F to 3H. After the second stack 350 has been formed, the hardmask layer may be removed, e.g., through a polishing process, and thestructure of FIG. 3H may be obtained.

The present disclosure may be further appreciated with the descriptionof the following embodiments:

In an embodiment, an integrated circuit includes a substrate thatincludes a first substrate region and a second substrate region. Theintegrated circuit also includes a first device over the first substrateregion and a second device over the second substrate region. The firstdevice includes a first plurality of nanowire strips of a firstsemiconductor material, a first gate structure surrounding at least oneof the first plurality of nanowire strips, and a first source/drainstructure contacting the at least one of the first plurality of nanowirestrips. The second device includes a second plurality of nanowire stripsof a second semiconductor material over the second substrate region, asecond gate structure surrounding at least one of the second pluralityof nanowire strips, and a second source/drain structure contacting theat least one of the second plurality of nanowire strips. The integratedcircuit also includes a semiconductor base layer embedded in one of thefirst substrate region or the second substrate region and under acorresponding one of the first device or the second device.

In a structure embodiment, a structure includes a substrate, a baselayer embedded in a recess portion in the substrate, a device over thesemiconductor base region. The base region includes a different materialfrom the substrate. The device includes a plurality of nanowire stripsof a semiconductor material, a gate structure surrounding at least oneof the plurality of nanowire strips, and a source/drain structurecontacting the at least one of the plurality of nanowire strips.

In a method embodiment, a first stack of a first plurality ofsemiconductor layers is formed over a crystalline substrate. A trench isformed within the first stack to expose the crystalline substratethrough the trench. A device spacer is formed adjacent to a sidewall ofthe trench. A recess portion is formed at a bottom of the trench throughan etching process that is anisotropic among crystalline facets of thecrystalline substrate such that the recess has a bottom portion and asloped sidewall portion that has an angle to the bottom portion. Asemiconductor base layer is formed within the recess portion. A secondstack of a second plurality of semiconductor layers is formed over thesemiconductor base layer.

The various embodiments described above can be combined to providefurther embodiments. All of the U.S. patents, U.S. patent applicationpublications, U.S. patent applications, foreign patents, foreign patentapplications and non-patent publications referred to in thisspecification and/or listed in the Application Data Sheet areincorporated herein by reference, in their entirety. Aspects of theembodiments can be modified, if necessary to employ concepts of thevarious patents, applications and publications to provide yet furtherembodiments.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

The invention claimed is:
 1. A method, comprising: forming a first stackof a first plurality of semiconductor layers over a crystallinesubstrate; forming a trench within the first stack to expose thecrystalline substrate; forming a device spacer adjacent to a sidewall ofthe trench; forming a recess portion at a bottom of the trench throughan etching process that is anisotropic among crystalline facets of thecrystalline substrate, the recess portion having a bottom portion and asloped sidewall portion that has an angle to the bottom portion; forminga semiconductor base layer within the recess portion; and forming asecond stack of a second plurality of semiconductor layers over thesemiconductor base layer.
 2. The method of claim 1, wherein the formingthe semiconductor base layer includes a pre-baking process.
 3. Themethod of claim 1, wherein the angle between the sloped sidewall portionto the bottom portion is between 5 degree and 89 degree.
 4. The methodof claim 3, wherein the crystalline substrate is silicon and the angleis within a range between 45 degree to 65 degree.
 5. The method of claim1, wherein an epitaxy process is used in the forming the semiconductorbase layer, the epitaxy process forming an epitaxial material of thesemiconductor base layer slower along a facet orientation of the slopedsidewall portion than along a facet orientation of the bottom portion.6. The method of claim 1, wherein the first stack includes a firstsilicon germanium layer and the second stack includes a second silicongermanium layer, the second silicon germanium layer includes a differentgermanium concentration than the first silicon germanium layer.
 7. Themethod of claim 1, wherein the first stack includes a first siliconlayer and the second stack includes a second silicon layer, the secondsilicon layer includes a different thickness from the first siliconlayer.
 8. A method of forming a semiconductor device, the methodcomprising: forming a first stack of alternating layers of a firstsemiconductor material and a second semiconductor material over acrystalline substrate; forming a trench within the first stack, whereina surface of the crystalline substrate is exposed in the trench;recessing a bottom of the trench to form a recess in the crystallinesubstrate, wherein the recess has sloped sidewalls along a first crystalplane of the crystalline substrate and a bottom surface along a secondcrystal plane of the crystalline substrate; filling the recess with abase semiconductor layer; forming a second stack of alternating layersof a third semiconductor material and a fourth semiconductor material;removing the alternating layers of the fourth semiconductor material;and forming a gate structure over the alternating layers of the thirdsemiconductor material.
 9. The method of claim 8 further comprising:prior to recessing the bottom of the trench, forming a spacer alongsidewalls of the trench.
 10. The method of claim 9, wherein the recessextends under the spacer.
 11. The method of claim 8, wherein the thirdsemiconductor material and the first semiconductor material are silicongermanium.
 12. The method of claim 11, wherein the third semiconductormaterial and the first semiconductor material have different germaniumconcentrations.
 13. The method of claim 12, wherein the secondsemiconductor material and the fourth semiconductor material aresilicon.
 14. The method of claim 8, wherein forming the basesemiconductor layer comprises: forming a base silicon germanium layer onthe crystalline substrate; and forming a base silicon layer over thebase silicon germanium layer, wherein the second stack of alternatinglayers is formed on the base silicon layer.
 15. The method of claim 8,wherein filling the recess comprises an epitaxial growth process inwhich a growth rate along the sloped sidewalls of the recess is lessthan a growth rate along a bottom of the recess.
 16. A method of forminga semiconductor device, the method comprising: forming a first stack ofalternating layers of a first semiconductor material and a secondsemiconductor material over a crystalline substrate; forming a trenchwithin the first stack, wherein a surface of the crystalline substrateis exposed in the trench; forming a dielectric layer along sidewalls anda bottom of the trench; removing the dielectric layer along the bottomof the trench to form a spacer along a sidewall of the trench and toexpose the crystalline substrate in the trench; etching the crystallinesubstrate to form a recess, wherein the recess has sloped sidewallsalong a first crystal plane of the crystalline substrate and a bottomsurface along a second crystal plane of the crystalline substrate;epitaxially growing a base semiconductor layer in the recess; forming asecond stack of alternating layers of a third semiconductor material anda fourth semiconductor material over the base semiconductor layer;removing portions of one of the alternating layers of the thirdsemiconductor material and the alternating layers of the fourthsemiconductor material; and forming a gate structure over remainingportions of the second stack of alternating layers.
 17. The method ofclaim 16, wherein epitaxially growing the base semiconductor layercomprises: epitaxially growing a first base layer; and epitaxiallygrowing a second base layer over the first base layer.
 18. The method ofclaim 17, wherein the first base layer and the third semiconductor layercomprise a same type of first material, and wherein the second baselayer and the fourth semiconductor layer comprise a same type of secondmaterial.
 19. The method of claim 18, wherein the same type of firstmaterial comprises silicon germanium, wherein a thickness of the firstbase layer is less than a critical thickness of the silicon germanium ofthe first base layer.
 20. The method of claim 18, wherein etching thecrystalline substrate to form the recess comprises exposing a lowersurface of the spacer.